$field should be the name of the field whose value you want to retrieve, as defined in https://www.cia.gov/library/publications/the-world-factbook/docs/notesanddefs.html. (WebService::CIA::Parser also creates four extra fields: "URL", "URL - Print", "URL - Flag", and "URL - Map" which are the URLs of the country's Factbook page, the printable version of that page, a GIF map of the country, and a GIF flag of the country respectively.)